The NSW Office of Environment and Heritage was ordered to pay nearly $200,000 in May after its Perisher sewage treatment plant was found to have been discharging toxic sewage into nearby waters for four months in 2017.

This water was used for snow guns at the nearby Perisher Blue ski resort within Kosciuszko National Park.

Perisher Blue, which runs the resorts, and the office, which runs the plant, agreed the elevated ammonia levels weren't a health risk, according to court documents.

The office was also required to pay $104,204 in legal costs to the Environmental Protection Authority, who prosecuted the case. The office was fined $84,000 for the incident which occurred from June 14, 2017 to September 4, 2017.

Perisher Blue referred The Canberra Times to the office when asked about the health concerns. The office said it had taken steps like adding more permanent staff at the site.

Court documents said there had been no noticeable impacts on animals using the waters, including macroinvertebrates.

NSW Land and Environment Court Judge Rachel Pepper's judgement revealed a lack of experienced staff, the sudden departure of an experienced plant operator at the start of the ski season and the adverse operating conditions in the Snowy Mountains contributed to the incident.

Colong Foundation director Keith Muir said the spill showed as the resorts got bigger, it was getting more difficult to manage their impact on the park.

"If these problems are returning all the time, then that suggests there should be no more development at all," Mr Muir said.

"It's straining the resources of the parks service which are inadequate to deal with it."

But he said managing a sewage treatment plant in a place with weather extremes like Kosciuszko was hard.

Because of the cold, the sewage needed to be constantly heated and mixed with the right balance of chemicals to treat it.

"It's like cooking," Mr Muir said.

National Parks Association NSW chief executive Gary Dunnett also said it was a hard job.

"I'm just really mindful of what an extraordinary challenge it is to deal with the huge influx," Mr Dunnett said. "The reality is that there's tens of thousands of people that visit the snow fields."

He said the 2017 incident was a sign the system was operating near its limits.

"It would need extraordinary scrutiny to make sure that any changes in that system weren't going to exceed capacity," Mr Dunnett said.

In February 2017, plant operators started a "test feed", which contained urea (urine), blood and bone. But problems were already surfacing.

On June 14, 2017 after the start of the ski season, nitrogen and ammonia levels above the allowable limits were detected in the sewage being discharged into the water.

Not long after the start of the ski season, one of the plant operators resigned on June 28. The staff who remained had not yet completed their plant operating training.

Exacerbated by the departure of the staff member, three private contractors were later brought in from Simmonds & Bristow, a water treatment company.

The system was unable to cope with the loads, mainly due to chemical imbalances because staff were unsure what levels of what chemicals to introduce.

In July, ammonia levels almost four times the maximum limit were being detected in the sewage discharge.

That month, a private contractor reviewed analysis of the discharge before he offering suggestions as to why elevated chemical levels were being detected. Eventually, on September 6, 2017, nitrogen and ammonia levels were under allowable limits.

The office said it had since increased the number of permanent plant staff from three to four, ran additional training programs on plant operating and arranged information sharing sessions with other sewage treatment plants across the Snowys.

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